This invention relates to an image receiving material for thermal dye transfer processes in which the material has an intermediate layer and a dye receiving layer, and to a process for producing same.
Thermal dye transfer systems, e.g. dye diffusion thermal transfer D2T2, produce images of originals by an electronic method. The originals are broken down into the primary colors cyan, magenta and yellow plus optionally black, which are then converted to electric pulses, and the pulses are relayed to a thermal printer where they are converted to heat in the print head.
The image receiving material passes through the thermal printer in contact with an ink donor element. In the print head the reverse side of a dye donor element is heated in accordance with the pulses given and a dye is released that diffuses or sublimes into this receiving layer of the image receiving material. This process is repeated successively for all the colors, then yielding the finished image.
It is known from JP-OS 60-236794 that carrier materials made of paper can be coated with high molecular thermoplastic materials before applying the image receiving layer. These high molecular thermoplastics may be polyolefins, polystyrene, polyvinylidene chloride, polyethylene terephthalate, polymethyl methacrylate or ionomer resins. This intermediate layer of thermoplastics should even out the irregularities in the paper surface.
U.S. Pat. No. 4,774,224 also describes an image receiving material for dye diffusion thermal transfer with a paper backing that is provided with a thermoplastic intermediate layer, preferably comprising a polyolefin. According to this patent, the surface roughness of the intermediate layer should be limited to &lt;0.2 .mu.m (average peak-to-valley roughness 7.5 .mu.inch).
European Patent No. 407 613 describes an image receiving material which also has thermoplastic intermediate layers. The thermoplastics are preferably polyolefins, polyvinyl chloride, polystyrene, polyethylene terephthalate, polymethacrylate or polycarbonate. The surface roughness of the intermediate layer is between 0.2 and 4.0 .mu.m in this patent.
All three patent publications describe image receiving materials for dye diffusion thermal transfer having a thermoplastic intermediate layer between the paper backing material and the image receiving layer. The quantity of the image transferred is supposed to be improved by the intermediate thermoplastic layer.
However, the image receiving materials of these three aforementioned publications have a definite shortcoming in the finished image with regard to thermal stability and aging resistance. The definition is progressively reduced, lines are broadened and blurred, and the text may be blurred to the point of illegibility. However, the thermal stability and the heat resistance of the image receiving materials are important criteria for their use and for suitability for use.
Therefore, the goal of this invention is to provide an image receiving material for dye diffusion thermal transfer processes that will make it possible to produce images with a high color density that are resistant to aging and heat, and will thus largely retain their good image quality.
This problem is solved by an image receiving material with an intermediate layer of a crosslinked material and an image receiving layer of uncrosslinked material applied to it, whereby both layers are produced from coating compositions that are free of organic solvents.
It was surprising to discover that through the combination of these two layers, a high color density and an excellent thermal stability and aging resistance of the image could be achieved because these good results could not be achieved either through use of two uncrosslinked layers or two crosslinked layers.
The materials used for the intermediate layer are lacquers of monomers, oligomers or prepolymers, but usually mixtures of these groups. Mainly the monomers serve as diluents in the lacquers. The monomers can be omitted to advantage if the coating compositions are processed at elevated temperatures, preferably 300.degree. C. to 60.degree. C.
The monomers, oligomers and prepolymers contain carbon double bonds (&gt;C.dbd.C&lt;), as acryl, methacryl, allyl or vinyl compounds. They may also contain hydroxyl groups, carboxyl groups and other polar groups, e.g. to improve adhesion of the image receiving layer.
Preferably, the crosslinkable compounds should comprise more than 50 wt % acrylate esters and/or methacrylate esters.
The lacquers can be filled to advantage before crosslinking with white pigments, such as carbonates, oxides, sulfates or sulfites of the elements calcium, magnesium, barium, strontium, zinc or titanium. Because of its high refractive index, titanium dioxide has proven especially suitable. Lacquers containing up to 70 wt % titanium dioxide have been processed successfully. Thus, the image background yields a high light reflection and makes the images appear more brilliant.
In addition, the lacquers may also contain up to 20 wt % additives, such as uncrosslinkable resins, optical brighteners matting agents, dyes and photoinitiators.
After applying the lacquer to the carrier material, it is crosslinked by means of high energy radiation which may be electron beam radiation or ultraviolet radiation. When using ultraviolet, photoinitiators must be added to the lacquer to form free radicals that initiate the crosslinking reaction.
The lacquers can be applied to the carrier material with the usual applicator systems, such as doctor blade or slit gap metering systems, grid rollers or multiple roll systems.
To produce especially brilliant images, the lacquer is brought in contact with high gloss metal surfaces, e.g. high-gloss cylinders, and crosslinked by exposing it to high energy electron radiation. The radiation treatment with accelerated electrons is applied from the back side, i.e. from the uncoated side of the carrier material. The electrons must be accelerated to the extent that their depth of penetration exceeds the thickness of the carrier material plus the lacquer layer. This technique is described in German Pat. No. 30 22 709.
To improve the flatness or dimensional stability of the carrier material, it may also be coated on both sides with a lacquer or precoated on one or both sides with thermoplastics, such as polyolefins.
A thermoplastic layer that has the function of a barrier layer beneath the intermediate layer of crosslinked lacquers prevents the penetration or absorption of the applied lacquer into the interior of the paper and, thus, saves on the use of lacquer material.
All suitable materials such as those known from the literature can be used for the image receiving layer. The finished image is resistant to aging and heat due to the intermediate layer. However, the coating composition should be free of organic solvents.
In order to assure good adhesion of the image receiving layer on the crosslinked intermediate layer, pretreatments such as corona discharge have proven suitable.